Given a string, s, consisting of letters and digits, generate all possible variations by modifying each letter independently to be either lowercase or uppercase while keeping the digits unchanged. Each letter in the s string can appear in both cases across different variations, resulting in multiple possible combinations. Return a list containing all such variations in any order.

Constraints:

• $1 \leq$ s.length $\leq12$

• s consists of lowercase English letters, uppercase English letters, and digits.